Phase lock loop applying in wireless communication system and method thereof

ABSTRACT

A phase lock loop receives a baseband signal which has an input frequency, and modulating the baseband signal to be a corresponding RF signal which has a predetermined transmission frequency for transmitting. The phase lock loop comprises a programmable divider, a modulator, a phase detector, a charging pump, a loop filter, a voltage-controlled oscillator and a frequency converter. The programmable divider divides the frequency of a local oscillating signal by a programmable divisor to generate a reference signal. The modulator receives the baseband signal, modulates the frequency of the reference signal according to the baseband signal, and generates a corresponding first comparison signal. The frequency converter receives the feedback RF signal and the local oscillating signal and outputs the second comparison signal according to the frequency difference. The divisor of the divider is programmable to avoid the spur frequency being generated because the local oscillating signal is interfered.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This present invention relates to a phase lock loop, and more particularly, the present invention relates to a phase lock loop applied in wireless communication systems.

[0003] 2. Description of the Prior Art

[0004] In a wireless communication system, a phase lock loop is used to generate a transmission signal that has specific phase and specific frequency. In a conventional phase lock loop, the specific frequency is generated through the following steps: first, generating a local oscillating frequency by a frequency synthesizer; next, dividing the local oscillating frequency by a fixed divider and inputting the signal with a reduced frequency into the phase detector; then, generating a control signal; and finally, generating the specific frequency by the control signal. Therefore, for the same specific frequency, the corresponding local oscillating frequency is fixed. However, because of the noise interference, the signal quality of the corresponding fixed local oscillating frequency becomes worse, and the signal quality of the transmission signal of the phase lock loop is also worsened.

[0005] There are several kinds of frequency synthesizer, but each of them has different kinds of noise interferences. For example, the interference caused by the multiplied frequency of the crystal oscillator frequency, that is when the desired local oscillating frequency approaches the multiples of a certain spur frequency, the local oscillating frequency is easily interfered. Thus, around the desired local oscillating frequency from several kHz to 1 MHz, there will be some interference signals.

[0006] The conventional method for preventing the noise interference focused on the improvement of the frequency synthesizer, but that will make the circuit more complicated and increase the cost of production. When the problem of interference remains unsolved, that frequency might not be usable, or that frequency might have a lower usage quality.

SUMMARY OF THE INVENTION

[0007] An objective of the invention is to provide a phase lock loop that applies in wireless communication systems and a method for generating a RF signal. The phase lock loop is used to receive a baseband signal that has an input frequency and modulate the baseband signal to be a corresponding RF signal. The RF signal has a predetermined transmission frequency for transmitting.

[0008] According to a preferred embodiment of the present invention, the phase lock loop comprises a frequency synthesizer, a first programmable divider, a modulator, a voltage-controlled oscillator, and a frequency converter. The frequency synthesizer is used to generate a local oscillating signal that has a local oscillating frequency. The first programmable divider is used to divide the frequency of the local oscillating signal by a first programmable divisor to generate a reference signal. The modulator is used to modulate the frequency of the reference signal according to the baseband signal to generate a corresponding first comparison signal. The voltage-controlled oscillator (VCO) is used to generate the corresponding RF signal for transmitting according to the corresponding control voltage generated by the phase difference between the first comparison signal and a second comparison signal. The RF signal is fed back as a feedback signal. The frequency converter is used to receive the feedback signal and the local oscillating signal and output the second comparison signal to the phase detector in responsive to the frequency difference of the feedback signal and the local oscillating signal. The first programmable divisor of the first programmable divider is programmable-controlled so as to prevent the occurrence of a spur frequency in the RF signal, besides the predetermined transmission frequency, due to the interfered local oscillating signal.

[0009] According to the phase lock loop of the present invention, the first programmable divisor of the first programmable divider is programmable-controlled so as to prevent the occurrence of a spur frequency in the RF signal, besides the predetermined transmission frequency, due to the interfered local oscillating signal. By using the present invention, all channels of the global system for mobile communication (GSM) can select a suitable local oscillating frequency, without having problems of low quality output signals.

[0010] The advantage and spirit of the invention may be understood by the following recitations together with the appended drawings.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

[0011]FIG. 1 is a function block diagram of a phase lock loop according to the present invention.

[0012]FIG. 2 is a function block diagram of a phase lock loop according to another preferred embodiment of the present invention.

[0013]FIG. 3 is an experimental data curve of the transmission frequency as the first programmable divisor M of the phase lock loop 30, shown in FIG. 2, is equal to 11.

[0014]FIG. 4 is an experimental data curve of the transmission frequency as the first programmable divisor M of the phase lock loop 30, shown in FIG. 2, is equal to 9.

[0015]FIG. 5 is an experimental data curve of the transmission frequency modulated by the baseband signal shown in FIG. 3.

[0016]FIG. 6 is an experimental data curve of the transmission frequency modulated by the baseband signal shown in FIG. 4.

[0017]FIG. 7 is a flow chart of the generating method of the RF signal of the phase lock loop shown in FIG. 2.

[0018]FIG. 8 is a function block diagram of a phase lock loop according to another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention provides a phase lock loop, which applies in the RF signal transmission device of a wireless communication system, for receiving a baseband signal that has an input frequency (F_(i)) and modulating the baseband signal to be a corresponding RF signal (F_(tx)); the RF signal has a predetermined transmission frequency for transmitting.

[0020] Referring to FIG. 1, FIG. 1 is a function block diagram of a phase lock loop 10 according to the present invention. The phase lock loop 10 comprises a frequency synthesizer 12, a first programmable divider 14, a modulator 16, a phase detector 18, a charging pump 20, a loop filter 22, a voltage-controlled oscillator 24, and a frequency converter 26.

[0021] The frequency synthesizer 12 is used to generate a local oscillating signal that has a local oscillating frequency F_(LO). The first programmable divider 14 divides the frequency of the local oscillating signal by a first programmable divisor M to generate a reference signal S_(re). The modulator 16 modulates the frequency of the reference signal S_(re) according to the baseband signal to generate a corresponding first comparison signal S₁.

[0022] The phase detector 18 is used to detect phases of the first comparison signal S₁ and a second comparison signal S₂ and output a corresponding current-controlled I/O signal S_(IQ) in response to the phase difference between the two comparison signals. The charging pump 20 is used to receive the current-controlled I/O signal S_(IQ) and accordingly output a corresponding control current I. The loop filter 22 filters the control current I to output a control voltage V to the voltage-controlled oscillator 24.

[0023] The voltage-controlled oscillator 24 generates the corresponding RF signal for transmitting, according to the control voltage V. The RF signal is fed back as a feedback signal S_(FB). The frequency converter 26 receives the feedback signal S_(FB) and the local oscillating signal to output the second comparison signal S₂ to the phase detector 18 in response to the frequency difference between the feedback signal S_(FB) and the local oscillating signal.

[0024] The first programmable divisor M of the first programmable divider 14 is programmable-controlled so as to prevent the occurrence of a spur frequency, besides the predetermined transmission frequency, in the RF signal due to the interfered local oscillating signal.

[0025] The following paragraphs describe how to prevent the local oscillating signal from being interfered by the first programmable divisor M that is programmable-controlled.

[0026] When the transmission frequency of the RF signal of the phase lock loop 10 is F_(tx), the relative equation between F_(LO) and the first programmable divisor M is $F_{LO} = {\frac{\left( {M - 1} \right)}{M} \times {F_{tx}.}}$

[0027] F_(tx) is a fixed value, and the conventional first programmable divisor M is also a fixed value. If the present result F_(LO) is interfered by the other signal, the conventional method of changing the circuit can not solve the interference problem effectively. Therefore, if the first programmable divisor M is programmable-controlled, another F_(LO) can be calculated to complete the phase lock loop 10 and obtain the same F_(tx).

[0028] For example, the global system for mobile communication (GSM) uses the channel 631 as the signal transmission channel. The frequency of this channel is 1734 MHz. If the programmable divisor N is fixed as 9, from the equation ${F_{LO} = {\frac{\left( {M - 1} \right)}{M} \times F_{tx}}},$

[0029] F_(LO) can be calculated as 1950.75 MHz. However, the frequency 1950.75 MHz is very easily interfered by the multiple frequency of the reference frequency 13 MHz or 26 MHz that are often used by the frequency synthesizer of the global sim mobile system. In this situation, the first programmable divisor is changed to 11, and then F_(LO) becomes 1907.41 MHz. The frequency 1907.4 MHz is 43.35 MHz apart from the frequency 1950.65 MHz, and the frequency 1907.4 MHz can prevent the interference from the multiple frequency of the reference frequency 13 MHz or 26 MHz.

[0030] Referring to FIG. 2, FIG. 2 is a function block diagram of a phase lock loop 30 according to another preferred embodiment of the present invention. Comparing the phase lock loop 10 in FIG. 1 with the phase lock loop 30 in FIG. 2, the phase lock loop 30 further comprises a phase shift generator 32 and a second programmable divider 34. The phase shift generator 32 is used to shift the phase of the reference signal S_(re) generated by the first programmable divider 14 by 90 degrees, and then input the phase-shifted reference signal into the modulator 16. Before the local oscillating signal enters the frequency converter 26, the frequency of the local oscillating signal is first divided by the second programmable divisor N in the second programmable divider 34, and then the frequency-divided local oscillating signal enters the frequency converter 26.

[0031] In the phase lock loop 30, the transmission frequency F_(tx) of the RF signal, the local oscillating frequency F_(LO) of the local oscillating signal, the first programmable divisor M of the first programmable divider 14, and the second programmable divisor N of the second programmable divider 34 satisfy the following equation: $F_{tx} = {\left( \frac{M \pm N}{M \times N} \right) \times F_{LO}}$

[0032] By this method, for the transmission frequency F_(tx) of the fixed RF signal of the phase lock loop, the better local oscillating frequency F_(LO) can be selected by changing the first programmable divisor M of the first programmable divider 14 and the second programmable divisor N of the second programmable divider 34, so as to avoid interfering the output signal of the phase lock loop by the specific local oscillating frequency F_(LO).

[0033] The following takes the experiment data of a preferred embodiment as an example to prove the effect of the present invention in real practice. The transmission frequency F_(tx) of the embodiment is 1725 MHz. A local oscillating frequency F_(LO) synthesized by the frequency synthesizer 12 of the phase lock loop 30 is 1897.5 MHz, and the reference signal S_(re) is generated by the first programmable divider 14. When the baseband signal for modulating the reference signal S_(re) has not been inputted, the output spectrum of the transmission frequency F_(tx) of the phase lock loop 30 is shown in FIG. 3. FIG. 3 is an experimental data curve of the transmission frequency as the first programmable divisor M of the phase lock loop 30, shown in FIG. 2, is equal to 11. The horizontal axis in FIG. 3 represents the frequency, and the vertical axis represents the power, wherein each unit in the horizontal axis means 300 kHz. The maximum power of the transmission frequency F_(tx) is the predetermined transmission channel, meaning that the transmission frequency F_(tx) is 1725 MHz. From FIG. 3, when the first programmable divisor M of the first programmable divider 14 of the phase lock loop 30 is equal to 11, the maximum output power of the transmission frequency F_(tx) is the apex A₁, 1725 MHz. However, the spur frequencies A₂ and A₃, not equal to 1725 MHz, also appear on the two sides of the frequency 1725 MHz at 500 kHz. Because the reference frequency 26 MHz used by the frequency synthesizer has the multiple frequency 1898 MHz (26×73=1898), and the multiple frequency is very close to the local oscillating frequency 1897.5 MHz, the interference occurs. After the interference passes the phase lock loop, the spur frequency A₂ and A₃ are generated on the two sides of the transmission frequency F_(tx) 1725 MHz at about 500 kHz.

[0034] Because the local oscillating signal corresponded to the transmission frequency F_(tx) 1725 MHz divided by the first programmable divisor 11 interferes the transmission frequency F_(tx), the first programmable divider 14 further adjusts the first programmable divisor M to 9, and the corresponded local oscillating frequency is changed to 1940.625 MHz. This frequency 1940.625 MHz is far away from another multiple frequency 1950 MHz (26×75=1950). Referring to FIG. 4, FIG. 4 is an experimental data curve of the transmission frequency as the first programmable divisor M of the phase lock loop 30, shown in FIG. 2, is equal to 9. From FIG. 4, when the first programmable divisor is adjusted to 9, the maximum output power B₁ of the transmission frequency F_(tx) is still 1725 MHz, but no spur frequency appears on the two sides of the frequency 1725 MHz any more.

[0035] A local oscillating frequency F_(LO) synthesized by the frequency synthesizer 12 of the phase lock loop 30 is 1897.5 MHz, and the reference signal S_(re) is generated by the first programmable divider 14. The output spectrum of the transmission frequency F_(tx) of the phase lock loop 30, after the reference signal S_(re) is modulated by the inputted baseband signal, is shown in FIG. 5. FIG. 5 is an experimental data curve of the transmission frequency F_(tx) modulated by the baseband signal shown in FIG. 3. The horizontal axis in FIG. 5 represents the frequency, and the vertical axis represents power, wherein each unit on the horizontal axis means 300 kHz. The maximum power of the transmission frequency F_(tx) is also the predetermined transmission channel, meaning that the transmission frequency F_(tx) is 1725 MHz. As shown in FIG. 5, when the first programmable divisor is equal to 11, and after modulated by the baseband signal, the spur frequency C₂ and C₃ appear on the two sides of the main transmission frequency C₁ in the output spectrum of the transmission frequency F_(tx).

[0036] Referring to FIG. 6, FIG. 6 is an experimental data curve of the transmission frequency modulated by the baseband signal shown in FIG. 4. When the first programmable divisor is adjusted to 9, there is no spur frequency on the output spectrum of the transmission frequency F_(tx), except the main transmission frequency D₁. Therefore, the first programmable divider of the present invention can effectively solve the problem of the transmission frequency F_(tx) when the local oscillating frequency is interfered by a specific multiple frequency and further eliminate the noise in the RF signal. Referring to FIG. 7, FIG. 7 is a flow chart of the generating method of the RF signal of the phase lock loop 30 shown in FIG. 2. According to the present invention, the RF signal is generated where the phase lock loop 30 receives a baseband signal that has an input frequency Fi, and modulates the baseband signal to be a corresponding RF signal. The RF signal has a predetermined transmission frequency F_(tx) for transmitting. The method comprises the following steps:

[0037] S40: Start;

[0038] S42: Generating the local oscillating signal with the local oscillating frequency F_(LO) and dividing the frequency of the local oscillating signal by the first programmable divisor M to generate the reference signal S_(re);

[0039] S44: Modulating the reference signal S_(re) according to the baseband signal Fi to generate a corresponding first comparison signal S1, after the phase of the reference signal S_(re) is shifted by 90 degrees;

[0040] S46: Dividing the local oscillating frequency F_(LO) by the second programmable divisor N and generating the second comparison signal S₂ according to the difference between the feedback signal S_(FB) and the local oscillating frequency F_(LO);

[0041] S48: Detecting the phases of the first comparison signal S₁ and the second comparison signal S₂ to output a corresponding current-controlled I/O signal S_(IQ) corresponding to a phase shift of two signals;

[0042] S50: Receiving the current-controlled I/O signal S_(IQ) to generate a corresponding control current and filtering the control current I to output a control voltage V;

[0043] S52: Generating the corresponding RF signal to be transmitted according to the control voltage V, with the RF signal as a feedback signal S_(FB) by feeding back;

[0044] S54: End.

[0045]FIG. 8 is a function block diagram of a phase lock loop according to another preferred embodiment of the present invention. The embodiment 50 illustrated in FIG. 8 is similar to the embodiment 30 discussed in FIG. 2, but adds a first filter 36, a second filter 38, a third filter 40, and a fourth filter 42 in some specific places to perform signal filtration for better signal quality and for improving overall performance. For simplified illustration, the same element used in both of the FIG. 2 and FIG. 8 would have the same numerical reference, and would not be explained again to avoid redundancy. Before entering the frequency converter 26, the frequency-divided local oscillating signal from the second programmable divider 34 is first filtered by the filter 36. After leaving the frequency converter 26, the signal is also filtered by the filter 38 and then goes to the phase detector 18. Similarly, the signal from the first programmable divider 14 is filtered by the filter 40, and is then inputted to the phase shift generator 32. The modulated signal from the modulator 16 would also go to the filter 42 first, and then go on to the phase detector 18. It is to remind that filters 36, 38, 40, 42 are not necessary at all times, and are provided for better signal quality and overall performance. Persons skilled in the signal processing and filtration field know in what situation a filter should be employed and what kind of filters is to be utilized.

[0046] The present invention provides a phase lock loop for receiving a baseband signal that has an input frequency and modulating the baseband signal to be a corresponding RF signal. The RF signal has a predetermined transmission frequency for transmitting. The phase lock loop comprises a frequency synthesizer, a first programmable divider, a modulator, a voltage-controlled oscillator, and a frequency converter. The first programmable divisor of the first programmable divider is programmable-controlled so as to prevent the occurrence of a spur frequency, besides the predetermined transmission frequency, in the RF signal due to the interfered local oscillating signal. By using the present invention, all channels of the GSM system, can select suitable local oscillating frequency, without causing the problem of lower output signal quality.

[0047] With the example and explanations above, the features and spirits of the invention will be hopefully well described. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A phase lock loop for receiving a baseband signal having an input frequency and modulating the baseband signal to be a corresponding RF signal having a predetermined transmission frequency for transmitting, the phase lock loop comprising: a frequency synthesizer for generating a local oscillating signal having a local oscillating frequency; a first programmable divider for dividing the frequency of the local oscillating signal by a first programmable divisor to generate a reference signal; a modulator for modulating the frequency of the reference signal according to the baseband signal to generate a corresponding first comparison signal; a phase detector for detecting phases of the first comparison signal and a second comparison signal, and outputting a corresponding current-controlled I/O signal in responsive to the phase difference of two comparison signals; a charging pump for receiving the current-controlled I/O signal and accordingly outputting a corresponding control current; a loop filter for filtering the control current to output a control voltage; a voltage-controlled oscillator (VCO) for generating the corresponding RF signal for transmitting according to the control voltage, the RF signal being fed back as a feedback signal; and a frequency converter for receiving the feedback signal and the local oscillating signal to output the second comparison signal to the phase detector in responsive to the frequency difference of the feedback signal and the local oscillating signal; wherein the first programmable divisor of the first programmable divider is programmable-controlled so as to prevent, except the predetermined transmission frequency, occurrence of a spur frequency in the RF signal due to the interfered local oscillating signal.
 2. The phase lock loop of claim 1, wherein the phase lock loop further comprises a phase shift generator for shifting the phase of the reference signal of the first programmable divider by 90 degrees, and then inputting the phase-shifted reference signal into the modulator.
 3. The phase lock loop of claim 1, wherein, before the local oscillating signal enters the frequency converter, the frequency of the local oscillating signal is first divided by the second programmable divisor in the second programmable divider, and then the frequency-divided local oscillating signal enters the frequency converter.
 4. The phase lock loop of claim 3, wherein, the phase lock loop further comprises a phase shift generator for shifting the phase of the reference signal of the first programmable divider by 90 degrees, and then inputting the phase-shifted reference signal into the modulator.
 5. The phase lock loop of claim 4, wherein, the phase lock loop further includes a first filter to filter the signal from the second programmable divider, a second filter to filter the signal leaving the frequency converter, the third filter to filter the signal from the first programmable divider, and a fourth filter to filter the modulated signal from the modulator.
 6. The phase lock loop of claim 3, wherein the transmission frequency of the RF signal is F_(tx), the local oscillating frequency of the local oscillating signal is F_(LO), the first programmable divisor of the first programmable divider is M, the second programmable divisor of the second programmable divider is N, and F_(tx), F_(LO), M, and N satisfy the following equation: $F_{tx} = {\left( \frac{M \pm N}{M \times N} \right) \times F_{LO}}$


7. The phase lock loop of claim 1, wherein the phase lock loop is utilized in a RF signal transmission device of a wireless communication system.
 8. The phase lock loop of claim 1, wherein the phase lock loop merely comprises the only frequency synthesizer to generate the single local oscillating frequency of the local oscillating signal.
 9. A method for generating a RF signal by utilizing a phase lock loop to receive a baseband signal having an input frequency and to modulate the baseband signal to be a corresponding RF signal for further transmitting in a predetermined transmission frequency, the method comprising the following steps: generating a local oscillating signal having a local oscillating frequency, and dividing the frequency of the local oscillating signal by a first programmable divisor to generate a reference signal; modulating the reference signal according to the baseband signal to generate a corresponding first comparison signal; detecting the phases of the first comparison signal and a second comparison signal to output a corresponding current-controlled I/O signal in responsive to the phase difference of two comparison signals; receiving the current-controlled I/O signal to generate a corresponding control current, and filtering the control current to output a control voltage; and generating the corresponding RF signal for transmitting according to the control voltage, the RF signal being fed back as a feedback signal; and generating the second comparison signal according to the frequency difference between the feedback signal and the local oscillating signal; wherein the programmable divisor is programmable-controlled so as to prevent, except the predetermined transmission frequency, occurrence of a spur frequency in the RF signal due to the interfered local oscillating signal.
 10. The method of claim 9, wherein the phase of the reference signal needs to be shifted by 90 degrees first, and then be modulated by the baseband signal.
 11. The method of claim 9, wherein the frequency of the local oscillating signal is divided by the second programmable divisor, and then the frequency difference is calculated by comparing the local oscillating signal with the feedback signal to generate the second comparison signal,
 12. The method of claim 11, wherein the phase of the reference signal needs to be shifted by 90 degrees first, and then to be modulated by the baseband signal.
 13. The method of claim 9, wherein the transmission frequency of the RF signal is F_(tx), the local oscillating frequency of the local oscillating signal is F_(LO), the first programmable divisor of the first programmable divider is M, the second programmable divisor of the second programmable divider is N, F_(tx) and F_(LO) satisfy the following equation: $F_{tx} = {\left( \frac{M \pm N}{M \times N} \right) \times F_{LO}}$


14. The method of claim 9, wherein the method is applied in a RF signal transmission device of a wireless communication system.
 15. The method of claim 9, wherein the method merely comprises the only frequency synthesizer to generate the local oscillating signal.
 16. The method of claim 9, wherein at least one filter is employed to filter the signals in the phase lock loop. 